This invention relates to metal-resin-metal sandwich laminates suitable for use in working, e.g. press forming with 180.degree. bending. More particularly, it relates to metal-resin-metal sandwich laminates which can be subjected to severe bending and drawing with no cracks in the outer metal layers and with minimized springback of the worked laminates.
Metal-resin-metal sandwich construction panels or sandwich laminates which comprise a resin core layer sandwiched between and bonded to outer metal layers are characterized by their excellent heat insulating and vibration damping properties as well as their light weight. In view of these characteristics, they were developed initially for use as building materials and, in fact, they have heretofore been employed mainly in such applications. Recently, however, the use of these sandwich laminates in the fabrication of various parts of automobiles and other vehicles has been studied with particular attention to their light weight. In such applications, the laminates are required to have good workability because they undergo relatively severe working such as bending and drawing during fabrication into the desired articles. Specifically, in these applications, the edge of a laminate is often bent to 180.degree. with a very small diameter nearly equal to its thickness in order to secure it to another sheet or laminate by lock seaming as shown in FIG. 2. Therefore, it is important for laminates for use in the fabrication of automobile parts such as automobile bodies to be capable of withstanding severe 180.degree. bending. Unfortunately the construction of the prior art metal-resin-metal sandwich laminates which have generally been employed as building materials or the like is not suited at all to undergo severe working since the outer metal layers bonded to the resin core layer tend to come away from the resin layer by the force applied thereon during bending, resulting in partial or complete delamination. In order to overcome this problem, it is known that the bond strength between a resin and a metal layer in a laminate can be retained even after bending when the resin is modified so as to have good adhesion to the metal. However, when certain modified resins such as modified polypropylene are employed as the resin core layer, they offer another problem in that cracking of the outer metal layers tends to occur during bending. Therefore, such laminates also cannot withstand severe working.
Many techniques have been proposed in the prior art to avoid deterioration in workability of metal-resin-metal sandwich laminates or to improve workability thereof. For example, Japanese Patent Publication No. 4739/1964 discloses specific requirements which ensure that a laminate will not spring back after bending and that delamination and breaking of the resin sheet will not occur during bending. It is proposed in Japanese Patent Publication No. 30111/1972 that powdered inorganic material be incorporated in the resin layer to improve cold workability of the laminate. However, neither of these prior art techniques gives special consideration to cracking of the outer metal layers during bending and therefore it cannot be expected that they are effective to avoid such cracking.
U.S. Pat. No. 4,313,996 to Newman et al., the Japanese counterpart of which is Japanese Patent Laid-Open Specification No. 156052/1980, discloses metal-plastic-metal laminates capable of withstanding bending up to an angle of 90.degree.. However, in view of the severe working actually encountered in the manufacture of automobile parts, it cannot be said that these laminates are satisfactory for automobile applications.
We have made various experiments to study the reason why cracking of the outer metal layers occurs during working and found that this type of cracking is caused by necking of the resin layer at or adjacent to the interfaces (i.e., bonded surfaces) between the resin and metal layers during working. The necking phenomenon means that the resin has yielded under the stress applied thereto during working. It is expected, therefore, that the use of a resin material which does not exhibit any yielding behavior (i.e., which does not have an yielding point) as the resin layer is effective to prevent cracking of the metal layers during working. It was, in fact, found that if the resin used as the core layer had enough ductility to enable it to follow severe working but had no yielding point, cracking of the outer metal layers could be avoided during working.
However, these ductile resins which do not exhibit any yielding behavior, in general, have elastomeric properties and their elastic limits are usually high. Therefore, sandwich laminates comprising such resin as a resin core layer may tend to spring back to a relatively greater extent upon working and hence are not suitable for use in working in some applications. In addition, this type of resins often has low softening temperatures so that the laminates may no longer retain their good heat resisting properties characteristic of sandwich laminates.